Generally for a load like this a roto phase converter would be a good choice as both loads could be run from one of them, as having one load running while the other starts allows for even more load, but that could be a problem as the 30 amp load might not be calling for cooling at the time the 20 amp load start, remember any 3 phase motor load will also act as a fly wheel generator that will act to help additional loads on the missing phase, also I saw some worry about single phase loads and or 120 volt loads, this was not correct, as two of the phases are always passed through to the load when using an RPC, I have installed many of these and I have never seen one wired any different, so passing through a neutral will always work fine as long as these loads are on one of the phases that is passed through.
But to wire the RPC correctly will take some motor control wiring skills to add timer relays to the control circuit that will start the RPC before the compressor starts, using an added time delay in the control circuit of each cooling unit so the the RPC has time to spin up before the compressor starts other wise you will have to have the RPC running 24/7 which will add to their electric bill, I have used an RPC for a 3-phase air compressor quite a few times and using the above controls is a must if you want the most economical way to run one, fairly simple circuit as you will now use the pressure switch (or thermostat as in this case) to control a timer relay that has both a settable timed output and a non-timed output, using the non-timed output to bring the motor starter in to start the RPC and the timed output to start up the compressor, these contacts must be isolated as you want each unit to be controlled by its own thermostat but either will bring up the RPC before either can start its compressor, also you should use an auxiliary contact on the motor starter for the RPC to prevent either cooler from trying to start if the overloads happen to trip for any reason, you do not want the cooler or freezer to try to start up if you don't have the RPC running.
but here are some points to ponder:
you will have to price the cost of not only the RPC (roto phase converter) but a magnetic motor starter with the correct overloads set for the FLA of the RPC, and the additional timers for each cooling unit, if you plan to just let the RPC run 24/7 then a manual motor starter will all you will need, but expect to let them know that the RPC will use as much as 50% of its FLA, also there is the requirements of a disconnect ahead of the RPC and even a 3-phase breaker panel or disconnects after the RPC for each load.
If needed I can draw you up a simple diagram for the wiring of the timer relays to control the RPC motor starter with each thermostat if you need. hopefully the two walk in's will be in close proximity to each other to keep the wiring cost down.
To size the RPC you will need the actual FLA of largest compressor as you do not need to size an RPC for both just the largest one, you might need to have a delay on one of them to keep both compressors from ever trying to start at the same time with the delay always allowing the larger load to start first so the delay should be placed on the smaller unit, (a little trick I use when I have two central airs on one whole house generator) once either unit has started it doesn't matter if the other one comes on line as either unit will act as a fly wheel to help the other to start.
If anyone sees any problem with the above please chime in.
The problem is this is not going to be cheap, and maybe changing the compressors to single phase units might be a viable choice given the above things you will have to go through to keep the long term cost down.
